1. Field of the Invention
This invention relates to analog-to-digital converters adapted to receive an electrical analog input signal and to produce a corresponding electrical digital output signal. More particularly, this invention relates to a successive-approximation A/D converter constructed in integrated circuit (IC) form.
2. Description of the Prior Art
Analog-to-digital converters of various types have been in use for many years, typically employed for converting analog measurements and the like into correspdonging digital signals for processing by high-speed digital computers. For certain applications, there has been considerable use of converters incorporating electronic ramp-signal integrators with clock-pulse timing devices for producing a digital count corresponding to the magnitude of an analog signal. A converter of the latter type having important advantageous features is disclosed in U.S. Pat. No. 3,872,466 issued to Ivar Wold on Mar. 18, 1975.
For other applications, there has been widespread use of so-called successive-approximation converters. Such converters include a digital-to-analog converter (often called a DAC) which during the conversion cycle is sequenced through a predetermined algorithm whereby the DAC output at appropriate stages is compared with the analog input signal to determine whether a corresponding bit of the final digital output signal should be "high" or "low". The results of this determination are used to set the respective stages of the successive-approximation register (SAR). The nature of such operation is in general well known, and is described for example at Page II-81 of the "A D Conversion Handbook" published by Analog Devices, Inc. of Norwood, Mass. Still further information on various converters may be found in the book "Electronic Analog/Digital Conversions" by H. A. Schmid (Van Nostrand Reinhold, 1970).
Among the important requirements for interface devices such as analog-to-digital converters is that they be small in size and economical to manufacture. Although such an objective has been suitably realized in a-to-d converters of the ramp-signal integrator type by constructing the converter as an integrated circuit on one or two monolithic chips, there has been no comparable advance achieved in the development of successive-approximation converters. In part, this has been due to the difficulty of using conventional processing technology to place on one or two chips a complete DAC together with all of the circuitry required for the successive approximation functions for controlling the DAC and storing the results of the successive analog comparisons.